1. Field of the Invention
The present invention is related to the field of geophysical exploration. More specifically, the present invention is related to the use of a seismic receiver in a wellbore, in particular for conducting a seismic survey while the wellbore is being drilled.
2. Discussion of the Related Art
A seismic receiver typically is deployed in a wellbore for determining the response of the earth to seismic energy in the vicinity of the wellbore, which enables determination of certain characteristics of the earth in the vicinity of the wellbore such as geological structure, and the location of changes in the material properties of the earth which may naturally occur.
The various uses of a seismic receiver deployed in a wellbore, also known as a borehole seismic receiver, are known in the art. For example, "Vertical Seismic Profiling", by Bob A. Hardage, Geophysical Press, London, 1985, describes typical applications for the use of a borehole seismic receiver.
One of the reasons for using a borehole seismic receiver is for matching various depths within the earth penetrated by the wellbore to specific travel times of seismic energy generated at the earth's surface. In relatively unexplored areas, geophysical surveys are typically conducted entirely at the earth's surface. Being able to determine the time for seismic energy to travel to a particular depth within the earth, using a surface seismic survey, depends on a portion of the seismic energy generated at the earth's surface for the survey being reflected from a zone in the earth having an acoustic impedance mismatch. Impedance mismatches, known as reflectors, typically occur at boundaries of changes in material composition or material properties of the earth. Reflectors are of particular interest for identifying possible exploration targets within the earth. In order to calculate the depth to a particular exploration target in the earth, where the seismic travel time is determined by the presence of a reflector in the surface seismic survey, it is necessary to determine the velocity of the seismic energy through the earth. The velocity of the seismic energy through the earth is strongly related to the composition and material properties of the earth. The material properties of the earth may vary widely within different earth formations within the depth range traversed by the wellbore.
It is not possible to explicitly determine velocity of the formations solely from the surface seismic survey, therefore, when a wellbore is drilled in a relatively unexplored area, a borehole seismic receiver is typically used to make measurements which are used to determine the velocity of the seismic energy within the earth.
Determining the velocity of the formations while the wellbore is being drilled, rather than after the drilling is completed, can be particularly valuable in certain instances. For example, some wellbores are drilled directionally to the exploration target because the target is located at a horizontally displaced location from the location of the wellbore at the earth's surface. If the target has been selected only on the basis of seismic travel time to a reflector, then the depth to the target may not be precisely determinable without knowledge of the velocity of the formations from the earth's surface to the depth of the target. This could cause the planned wellbore trajectory to miss the target entirely. Periodic use of a wellbore seismic receiver during drilling, in conjunction with a seismic energy source deployed at the earth's surface directly above the position of the wellbore seismic receiver, enables measurement of seismic energy travel time to the depth of the seismic receiver deployed in the wellbore. The measurement of seismic travel times to various depths enables calibration of the surface seismic survey travel time in depth, thereby increasing the probability that the target will be penetrated by the wellbore.
Certain reflectors observed on the surface seismic survey are of particular concern in drilling the wellbore. For example, "Abnormal Formation Pressure", by Walter Fertl, Elsevier Publishing, Amsterdam, 1976, describes reflectors which sometimes correlate to the presence of significant changes in the gradient of fluid pressure contained within some formations. Knowledge of the precise depth of the reflector could prevent drilling problems which might result from an unintended penetration by the wellbore of a formation containing fluid pressure with a significantly different gradient than the gradient otherwise expected in the vicinity of the wellbore. The use of a borehole seismic receiver to calibrate seismic travel time to the wellbore depth could enable more precise determination of the depth of the reflector, which could prevent unintended penetration of the formation having abnormal fluid pressure.
It is also known in the art to use borehole seismic receivers for generating seismic reflection sections in an area within about 1000 feet around the wellbore. Seismic energy from the seismic energy source also travels deeper than the receiver in the wellbore, and the seismic energy can be reflected by zones having acoustic impedance mismatch, just as with a surface seismic section. The reflection energy can be identified by appropriate processing of a recording of the energy detected by the receiver. The identified reflection energy can be displayed in a form for comparing the borehole seismic survey with the surface seismic survey.
It is difficult to use the borehole seismic receivers known in the art, while the wellbore is being drilled. Each time the borehole seismic receiver is to be run in the wellbore, drilling the wellbore must stop, and a drillpipe, which is used to operate a drilling bit, must be removed from the wellbore. The drillpipe is formed from sections each having a length of thirty to ninety feet. The sections are joined by threaded connections which must be uncoupled each time the drillpipe is removed from the wellbore. Further, the borehole seismic receiver must be run in the wellbore on an electrical cable, or wireline, and a clamping mechanism, which forms part of the borehole seismic receiver and forces the receiver into contact with the wellbore wall, is deployed to enable good acoustic coupling from the wellbore wall to the borehole seismic receiver. There is a significant risk of the borehole seismic receiver becoming stuck in the wellbore. Retrieving the borehole seismic receiver when it is stuck in the wellbore is particularly difficult because the wireline obstructs the process of retrieval of objects stuck in the wellbore.
It is an object of the present invention to provide a borehole seismic receiver which can be deployed in a wellbore without removing the drillpipe from the wellbore.
It is a further object of the present invention to provide a borehole seismic receiver which can be deployed without the use of an electric wireline.